Friday, May 23, 2014

El Niño Winter Temperatures

With a potentially strong El Niño taking shape, let's take a quick look at how temperatures in Alaska have responded to strong El Niños in the past. To define a strong El Niño, I used the Value of the Niño 3.4 region. A chart of historical values is shown in Figure 1. There are 8 events that crossed the 'Strong' threshold. Those 8 events were used in my subsequent analysis.

The first pass at looking for temperature patterns was the ESRL reanalysis site. I queried the December, January, February temperature anomalies for each of the years with a strong El Niño. Figure 2 shows the results.

A pattern is clearly apparent. El Niño winters are markedly cooler in the northwest part of the state and slightly above normal in the Panhandle. End of story, right? Well, not exactly.

Remember that many stations in Alaska have seen their temperatures rise over the course of several decades. In some cases quite substantially. There are a myriad of reasons for this (climate change, station relocations, urbanization, sea ice reduction, etc.) that are not relevant to the current discussion. The ESRL reanalysis always uses the 1981-2010 climate normal period when calculating anomalies; whether it is wind, pressure, heights, or temperatures. Since the 1981-2010 temperature normals are higher (warmer) than previous decades, the data shown in Figure 2 actually overstates the magnitude (but not the pattern) of El Niño's effect on temperatures.

Since I have calculated normal temperatures for the three major stations in Alaska using the trailing 30-year climate normal periods (1921-1950, 1931-1960, etc.), we can compare the wintertime difference from the current normal period and also the wintertime difference from the temporally relevant normal period. Figure 3 shows the result of that analysis.

Figure 3. Winter temperature departure (°F) during 8 strong El Niño events since 1950 for Juneau, Fairbanks, and Anchorage using the current normal period and the temporally relevant normal period.

The chart in Figure 3 is a little busy but basically it gives a side-by-side comparison of how each of the 8 winter temperatures compares to the wintertime normal period at the time and the current wintertime normal. The solid fill pattern is the temperature departure from the relevant 30-year climate normal period and the stippled pattern is the temperature departure from the 1981-2010 normal period. What stands out, particularly in the rightmost (Overall) category is how the strong El Niño temperatures are actually warmer than normal in comparison to the normals from the relevant time period. Using the current climate normal period gives a false impression of how the temperatures actually respond to El Niño conditions. Using the appropriate climate normal period changes the relationship from negative to positive.

Additional information:

Here is a paper by John Papineau, formerly with the Anchorage NWS Office, with a description of the effect of El Niño in Anchorage.

I would like to say that the huge negative temp departures through the seventies and the about-face to the present is because of the PDO. This especially in light of the posts on El Nino on Mar 27 and Apr 10. However, there has to be more. The PDO is not an answer for everything, is it?

And is there anything that has been posted on this website that is original enough to be published in a journal? Sure seems like it.

Thanks Eric. Yes, the PDO is much more relavent but on a macro scale. Since El Nino is in the news a lot I thought it would be interesting to look at it in isolation. More importantly, I wanted to demonstrate the perils of using an inappropriate climate normal period.

Brian, Nice job - we often rely on these rather weak analog signals, but you've shown that they can look completely different or even be of the opposite sign when using a contemporary climatology! This is an interesting discovery.

Eric - I think much of the material on this blog could be considered as seeds for detailed investigation that would lead to formal publication. The problem is that the publishing process is enormously arduous and time-consuming, and I certainly don't have any more spare time.